The present invention relates to trisector, rotary regenerative air preheaters employing double radial seals and more particularly to the arrangement and sizing of the sector plates which divided the preheater into the three sectors.
Pulverized coal firing is the most commonly used procedure for firing coal in large steam generators, such as utility steam generators. Pulverized coal firing normally utilizes air for drying, classification and transport of the coal in the pulverizer. The air to the pulverizer is referred to as primary air while the remaining combustion air is referred to as secondary air. It is normally required that the coal be dried before ignition can take place and this drying is accomplished by the use of hot primary air which then transports the dried pulverized coal to the furnace.
The trisector air preheater is used on large coal-fired boilers particularly where a cold primary air fan is desirable. The preheater is designed so that, by dividing the air-side of the preheater into two sectors, the higher pressure primary air may be heated along with the secondary air in a single air preheater.
It is well known to provide sealing means in air preheaters to prevent the mingling of the flue gas with the air. These may include axial seals around the outer periphery of the rotor between the rotor and the housing and radial seals which extend along the upper and lower edges of the radially extending partitions that form the compartments in the rotor. The radial sealing member along the edges of the partitions wipe against the sector plates which divide the air preheater into sectors for the gas and air. The engagement of these radial seals with the sector plates minimizes the leakage and the mixing of the gas and air.
In order to keep the leakage as low as practical, it is common to provide a double sealing arrangement. In this arrangement, the sector plates are equal in size to two rotor compartments so that the radial seals on two consecutive radially extending rotor compartment partitions (diaphragms) are in engagement with the sector plate at the same time. This contrasts with a single seal arrangement where the sector plates need only be equal in size to one compartment. The problem with a double seal arrangement is that the sector plates occupy or bock-off a significant percentage of the flow area through the air preheater. Each double seal sector plate blocks off twice as much flow area as a single seal sector plate. That means that the pressure drop through the air preheater is increased or that the size needs to be increased.